Method and device for determining characteristics of the flow of a medium

Abstract: The invention relates to a method for determining characteristics of the flow of a medium in a channel by measuring the transit time of sound waves which are transmitted between one or more transducers, and determining the characteristics from the measured transit times. In one embodiment of the method according to the invention, at least two sound waves whose transit time depends on swirl are transmitted along acoustic paths with a different sensitivity to swirl, and a measure of the swirl is determined from the measured transit times. In another embodiment, at least two sound waves whose transit time depends on the symmetry of the flow profile are transmitted along acoustic paths with a different sensitivity to symmetry, and a measure of the symmetry is determined from the measured transit times. These embodiments are preferably combined. If weighting factors are allocated to the values obtained, the individual velocities of the sound waves can be used to calculate the average flow velocity and/or the throughput of the medium. Since allowance is made for possible disturbances of the velocity profile, the accuracy of the calculated variables is good. The invention also provides for a device, by means of which the method according to the invention can be carried out.

Description: Generally the present invention relates to a method for determining swirl and symmetry of the flow of a medium in a channel by transmitting and receiving sound waves along two or more acoustic paths having different degrees of sensitivity to swirl and to symmetry using one or more acoustic transducers, each of which can act individually as a transmitter and/or receiver, and by measuring the transit times of the transmitted sound waves, and determining a value of swirl and symmetry from the measured transit times. Furthermore the invention relates to a device for carrying out this method. Acoustic flow measurements are already known. It involves determining the average flow velocity and/or the throughput of the medium from the difference in transit time of sound waves which are transmitted in the downstream direction and upstream direction respectively between acoustic transducers set up at a distance from each other. The medium can be a gas or liquid. The transit times of the sound waves not only depend on the flow velocity of the medium, but are also influenced by the flow profile of the medium. In a flowing medium, apart from the longitudinal movement, a swirl can occur. Other possible disturbances of the ideal flow profile are a flow velocity fluctuating in time and a flow profile which is asymmetrical relative to the longitudinal axis. Such disturbances occur in particular in pipe systems which have a complex structure. The ultimate reliability of the calculated flow velocity of the medium depends on the distance covered, the acoustic path, on the transmitted sound wave, and on the calculation method used. Many configurations are known for the acoustic path. In the case of the conventional methods which are used in commercially available measuring instruments, several acoustic paths are used, running parallel to each other. The known numerical Gaussian square method is used for positioning of the paths and the various weighting factors which are allocated to the measured velocities. The advantages of this method are clear. No additional information on the flow profile is required for calculating the velocity. The weighting factors are fixed in advance, so that the microprocessor which is used for calculation of the characteristics need only carry out a limited number of calculations. Although this means that measurement of the flow velocity is simple to carry out and excellent results can be obtained in ideal flow conditions, the method has a number of clear disadvantages, due to the underlying assumptions and limitations of the Gaussian integration method. These underlying assumptions and limitations are as follows: It is assumed that the flow profile is fully axially symmetrical. No additional information from the Reynolds number is used. As a result of the fixed weighting factors for the various acoustic paths, which are optimized for the undisturbed flow profile, errors will occur when the actual flow profile deviates from the ideal profile. In Acoustical Imaging, vol. 18, 1991 New York (US) in an article entitled "Ultrasonic time-of-flight tomography for non-intrusive measurements of flow velocity field" by Axel Hauck a method and apparatus are disclosed for determining the actual flow velocity. The apparatus disclosed comprises two tiltable arrays of 36 transducers, positioned on both sides of a conduit. The acoustic paths of the 36 pairs of associated transducers are straight lines between the transducers and cover the most part of the cross section of the conduit. Based on the measured transit times of the emitted sound waves the real flow profile can be calculated and by means thereof the flow velocity of the medium. Furthermore a method and apparatus for determining non-axial flow are described in the European Patent Application EP-A-0 588 599 (a document according to Art, 54(3) EPC), using two pairs of transducers having acoustic paths crossing each other in the center of the conduit. The object of the present invention is to provide an improved method for determining the swirl and symmetry in the flow of a medium in a channel. Another object of the present invention is to provide an improved method for determining the flow velocity and/or throughput of a medium in a channel with more accurate results using the above determination, due to the fact that errors caused by disturbances of the ideal flow profile are eliminated. The method and device for determining swirl and symmetry according to the invention are defined in independent claims 1 and 7 respectively. To provide acoustic paths with different sensitivities sound waves may be transmitted downstream and upstream. A measure of the swirl can be determined from comparison of the measured transit times of the transmitted sound waves along acoustic paths with different degrees of sensitivity to swirl. If the measured transit times are identical, there is no swirl. If the measured transit times are not identical, the difference in transit time is a measure of the intensity of the swirl. This measure of the intensity contains both the magnitude and the direction of the swirl. The channel in which the method is carried out will in many cases be a cylindrical pipe, but the method can also be used in channels of other shapes, such as a square cross-section or a U-shaped channel. The method is used in a cylindrical pipe through sound waves whose transit time depends on swirl being transmitted along various acoustic paths with at least two reflections against the wall of the channel, and at least one sound wave being transmitted clockwise and at least one sound wave in a direction opposite thereto to provide the different sensitivity acoustic paths. The sound waves, two reflections of which are taken up in the acoustic path, traverse a large part of the cross-section of the channel and thus give a reliable picture of the flow. For determining symmetry, sound waves are transmitted along acoustic paths which are sensitive to asymmetrical flow profiles. In a similar way to that described above for determining a measure of the swirl, a measure of the symmetry is derived from the difference in transit time of said sound waves. The sound waves whose transit time depends on the symmetry of the flow profile are transmitted along different acoustic paths with one reflection against the wall of the channel, in which case the acoustic paths cross the longitudinal axis of the channel. Such sound waves are preferably transmitted along three different acoustic paths, so that the sound waves traverse the entire cross-section of the channel. The method according to the invention first of all gives an insight into the possible presence and magnitude of disturbances of the ideal flow profile. Prior calibration is not necessary. For this purpose, two sound waves are transmitted along acoustic paths in the downstream and upstream direction respectively. Two sound waves are preferably transmitted along each acoustic path in the downstream and upstream direction respectively. The average flow velocity can be determined from the difference in transit time of sound waves transmitted along the same acoustic path, but in opposite directions of flow. In order also to detect the presence of a flow which fluctuates in time, several sound waves are transmitted in rapid succession along the same acoustic path, and a measure of the pulsation of the flow of the medium is determined from the measured transit times thereof. The difference in transit time of two sound waves transmitted in rapid succession along the same acoustic path is a measure of the fluctuation in time of the flow of the medium. The method is preferably carried out by allocating weighting factors to the measure of swirl and/or symmetry, and if desired the pulsation, and by means thereof and the individual velocities along the various acoustic paths calculating the flow velocity and/or the throughput of the medium in the channel. Since account is taken of possible disturbances when calculating the flow velocity and/or the throughput of the medium, a reliable value is obtained for them. The Reynolds number is advantageously involved in the calculation of the flow velocity and/or the throughput of the medium. The accuracy of such a measurement is better than 0.3% of the calculated value for flow velocities of 0.3 to 30 m/s (1 to 100 foot/sec). The individual velocities along the various acoustic paths can be determined by measuring the transit time of sound waves transmitted along the same path in the downstream and upstream directions respectively, as described above. Measurements by means of the method according to the invention can be carried out simply in complex pipe systems, without the flow of the medium having to be evened out by flow elements or other aids. The transducers transmitting sound waves whose transit time depends on swirl may transmit at least one sound wave in the clockwise direction, while at least one sound wave is in the opposite direction. With these devices according to the invention an insight can be gained into the type of flow of the medium. The transducers which transmit sound waves whose transit time depends on the symmetry of the flow profile transmit sound waves along various acoustic paths with one reflection against the wall of the channel, which acoustic paths cross the longitudinal axis of the channel. The device is preferably provided with three pairs of such transducers, so that the sound waves which are transmitted along the various acoustic paths scan the entire cross-section of the channel. A pair of transducers advantageously sends two sound waves along the same path in the downstream direction and upstream direction respectively to define the different acoustic paths, so that the means for determining the characteristics from the measured transit times can determine the flow velocity along said acoustic path. Each pair of transducers preferably transmits sound waves along each acoustic path in the downstream direction and upstream direction respectively. In order also to determine the measure of pulsation of the flow with the device according to the invention, the transducers can be designed to transmit several sound waves in rapid succession along the same acoustic path, and the means for determining the characteristics from the measured transit times also determine a measure of the pulsation. In order to permit use of the device as a flowmeter, the means for determining a value of swirl and symmetry calculate the flow velocity and/or the throughput of the medium from these values and the individual velocities along the various acoustic paths. These means advantageously calculate the flow velocity and/or the throughput of the medium from the measure of the swirl and symmetry, and if desired the pulsation and the individual velocities, along the various acoustic paths. The invention will be explained in greater detail below with reference to the appended drawing, in which: Fig. 1 is a projection of various acoustic paths of the transmitted sound waves according to the invention: Fig. 2 is a view of an acoustic path with a single reflection; and Fig. 3 is a view of an acoustic path with a double reflection. Fig. 1 shows a configuration of acoustic paths of the sound waves according to a preferred embodiment of the invention. In the situation shown, the channel is a cylindrical pipe 6. The reference numbers 1 A/B to 5A/B indicate pairs of transducers set up at a distance from each other, which transducers can act as transmitters and receivers. The transducers 1 A and 4A transmit sound waves (indicated by a solid line) along various acoustic paths with two reflections, a sound wave 11 being transmitted in an anticlockwise direction, and a sound wave 14 in a clockwise direction. The acoustic paths of the sound waves 11 and 14 have a different sensitivity to swirl in the flow. The transmitted sound waves are received by the transducers 1 B and 4B respectively. Said transducers 1 B and 4B themselves transmit sound waves (not shown) in the opposite direction along the acoustic paths with two reflections, which sound waves are received by the transducers 1A and 4A. The transmitted sound waves 11 and 14 and the sound waves transmitted in the opposite direction traverse a large part of the cross-section of the channel 6. The transit time of the sound waves between the transducers depends on swirl. The transducers 2A, 3A and 5A transmit sound waves 12, 13 and 15 respectively (shown by a dashed line) with a single reflection against the wall of the channel 6. The acoustic paths of the sound waves 12, 13 and 15 have a different sensitivity to the symmetry of the flow profile. The transducers 2B, 3B and 5B receive said sound waves and themselves transmit sound waves in the opposite direction along different acoustic paths. The sound waves cross the longitudinal axis of the channel 6. The transit time of said sound waves depends on the symmetry of the flow. Reference number 7 indicates diagrammatically means for measuring the transit time of the sound waves. Said means 7 are connected to the transmitters and receivers. Means 8 for determining the characteristics of the flow, such as the intensity of the swirl and asymmetry of the flow profile, are also shown diagrammatically Figs. 2 and 3 show acoustic paths with a single and double reflection respectively between a transmitter A and receiver B. The arrow v m indicates the direction of flow of the medium. Examples of the interpretation and processing of the measured transit times of sound waves with acoustic paths, as shown in Fig. 1, are given below. The results are presented in the form of a matrix. The table below is an example of such a matrix. The transit times of the sound waves transmitted in the same direction of flow along different acoustic paths which are sensitive to symmetry are compared. If there are deviations, a distinction is made in the size of the differences measured. The deviations are broken down into one or more of three categories, namely low, average and high. This means that in that case a low, average and high deviation of the symmetry in the flow is present. Of course, more or fewer categories may be used if desired. The transit times of the sound waves which are sensitive to swirl of the flow are also compared and, if there are deviations, they are broken down into one of the three categories. For each type of sound wave it is also ascertained whether the flow velocity fluctuates in time. The flow velocity of the medium is also calculated for each acoustic path from the difference in transit time of sound waves which are transmitted along the same path in the downstream and upstream direction. The flow velocity and/or the throughput are calculated with the aid of this matrix and weighting factors to be allocated to it and the individual velocities along the various acoustic paths. Two examples of flow profiles and the corresponding matrix are given below. Example 1 In Table 2 below the flow behaviour of a medium with an ideal flow profile is characterized. In this table "0" means absent and "X" means present. In an ideal flow profile there are no differences in the transit times of sound waves (downstream or upstream) along similar acoustic paths, and no fluctuations of the flow velocity in time occur. Example 2 Table 3 below shows the behaviour of a medium after a double bend in a compressor station. As can be seen from Table 3, the flow profile of a medium after a double bend in a compressor station is disturbed. Both asymmetry and swirl occur, so that the symmetry of the flow changes in time. From the measured transit times an average to high value is allocated to the intensity of the disturbance. The Reynolds number is also influenced by these disturbances. If weighting factors are allocated to the disturbances occurring, said weighting factors and the individual velocities along the acoustic paths can be used to calculate the average flow velocity and from it the throughput.

Claims: Method of determining swirl and symmetry in the flow of a medium in a channel (6) by transmitting and receiving first sound waves (11, 14) along two or more first acoustic paths having different degrees of sensitivity to swirl using first transducers (1A, 1B, 4A, 4B), each of which can act individually as a transmitter and receiver, which are arranged at positions on the periphery of the channel (6) such that each first acoustic path (11, 14) comprises at least two reflections against the wall of the channel (6) and by measuring the transit times of the transmitted first sound waves, and determining a value of swirl from the measured transit times, the method also comprising transmitting and receiving second sound waves (12, 13, 15) along two or more second acoustic paths having different degrees of sensitivity to symmetry using second transducers (2A, 2B, 3A, 3B, 5A, 5B), each of which can act individually as a transmitter and receiver, which are arranged at positions on the periphery of the channel (6) such that each second acoustic path (12, 13, 15) crosses the longitudinal axis of the channel (6) with one reflection against the wall of the channel (6), and measuring the transit times of the transmitted second sound waves and determining a value of symmetry of the flow profile from the measured transit times. Method according to claim 1, characterized in that at least one of the first sound waves is transmitted in the clockwise direction and at least one of the first sound waves is transmitted in the counterclockwise direction, to provide acoustic paths with different degrees of sensitivity. Method according to claim 1 or 2, characterized in that two sound waves are transmitted in the downstream and upstream direction respectively, thereby forming two acoustic paths with different degrees of sensitivity. Method according to one of the preceding claims, characterized in that several sound waves are transmitted in rapid succession along the same acoustic path, and a value of the pulsation of the flow of the medium is determined from the measured transit times thereof. Method according to one of the preceding claims, characterized in that weighting factors are allocated to the value of swirl and symmetry and optionally pulsation and by means thereof and the individual velocities along the various acoustic paths the flow velocity and/or throughput of the medium in the channel is calculated. Method according to claim 5, characterized in that the Reynolds number is involved in the calculation of the flow velocity and/or the throughput of the medium. Device for determining swirl and symmetry in the flow of a medium in a channel (6), the device comprising first acoustic transducers (1A, 1B, 4A, 4B), each of which is arranged to act individually as a transmitter and receiver and arranged to transmit sound waves (11, 14) along two or more first acoustic paths having different degrees of sensitivity to swirl, the first transducers being arranged at positions on the periphery of the channel (6) such that each first acoustic path thereof has at least two reflections against the wall of the channel (6) and means (7) for measuring the transit time of the transmitted first sound waves, as well as means (8) arranged to determine a value of swirl of the flow profile from the measured transit times, the device also comprising second acoustic transducers (2A, 2B, 3A, 3B, 5A, 5B), each of which is arranged to act individually as a transmitter and receiver and arranged to transmit sound waves (12, 13, 15) along two or more second acoustic paths having different degrees of sensitivity to symmetry, provided by transmitted second sound waves in the downstream and upstream direction respectively, the second transducers being arranged at positions on the periphery of the channel (6) such that each second acoustic path crosses the longitudinal axis of the channel (6) with one reflection against the wall of the channel (6), and the means for measuring (7) also measures the transit time of the transmitted second sound waves and the means (8) also determines a value of the symmetry of the flow profile. Device according to claim 7, characterized in that at least two first acoustic paths are provided by transmitted first soundwaves in the clockwise and counterclockwise direction. Device according to claim 7 or 8, characterized in that tranducers transmit sound waves in the downstream and upstream direction respectively, to provide acoustic paths with different degrees of sensitivity. Device according to one of claims 7-9, characterized in that the transducers (1A, 2A, 3A, 4A, 5A, 1B, 2B, 3B, 4B, 5B) are designed to transmit several sound waves in rapid succession in the same direction along the same acoustic path, and the means (8) for determining the swirl and/or symmetry from the measured transit times also determine a measure of the pulsation of the flow. Device according to one of claims 7-10, characterized in that the means (8) for determining the swirl, symmetry and/or pulsation calculate the flow velocity and/or the throughput of the medium from the value of swirl, and symmetry, and optionally pulsation and the individual velocities, along the various acoustic paths.

Inventors: DRENTHEN JAN GERT, DRENTHEN, JAN GERT
Applicants: INSTROMET ULTRASONICS BV, INSTROMET ULTRASONICS B.V.
Application year: 1994
Main classification: G01P5
IPC codes: G01F1, G01P5